156 



IMPORTANCE AND VALUE 



The great variety and number of manures dis- 

 played in this table may serve to dispel apprehen- 

 sion as to what is to happen when guano is ex- 

 hausted, and bone-dust and rape-meal in Germany 

 are insufficient to meet the demand. When this 

 period arrives, instead of fertilizing the soil with 

 the nitrogen of these manures, the nitrogen of pit- 

 coal, which is now in a large proportion un profita- 

 bly lost, will, as in England, be substituted for it. 

 And what, when this also is exhausted ? In that 

 event, we shall derive heat from water, and ma- 

 nure the ground with air. However chimerical this 

 assertion may sound, it is, nevertheless, far from 

 being improbable. For it has been already shown 

 that a kind of air, hydrogen gas, which is combus- 

 tible and possesses a heating power eight times 

 greater than that of wood, is obtained from the de- 



